A phase I/II trial was undertaken to determine the maximum tolerated dose of polyethylene glycol interferon-alpha-2b (PEG-IFN) with interleukin-2 (IL-2), and to evaluate the efficacy and toxicity in patients with metastatic renal cell carcinoma. Patients initially received subcutaneous PEG-IFN, 3.0 mcg/kg/wk, combined with IL-2, but owing to unexpected toxicity a revised phase I schedule ensued. Patients received 1.0, 1.5, 2.0, or 3.0 mcg/kg/wk of PEG-IFN on days 1, 8, 15, and 22; subcutaneous IL-2 was given at a dose of 5 x 10 IU/m2 every 8 hours x 3 on day 1, followed daily at 5 x 10 IU/m2 days 2, 3, 4, and 5 of week 1, then 5 times per week for 3 weeks, followed by 2 weeks off. The maximum tolerated dose of PEG-IFN was 2.0 mcg/kg/wk. Fifty-four patients were enrolled. Frequent grade III/IV cardiac and neurologic toxicities led to an expanded phase I trial. Eleven serious events in 33 patients in the phase II portion led to early termination. No patient died from treatment. The overall response rate in 53 evaluable patients was 30.2% (95% confidence interval 20.5-39.9), with 2 complete responses and 14 partial responses and at least 1 response at each dose level. The median duration of response was 11 months (range, 2 to 65+ mo); median survival was 20 months (range, 2 to 71+ mo); median time to progression was 4 months. Despite clinical efficacy, the study was closed prematurely owing to excess toxicity. Although all serious adverse events resolved, this degree of toxicity is unacceptable for an outpatient treatment regimen.